text:
Is the twin vs quad engine industry preference really challenged?
This question relates to this article at Leeham, essentially this article challenges the concept that twin engined airliners are more efficient than quad engined. However the article fails to show the data and prove the arguments. My question is asked to those who are able to confirm the arguments of the article. I will describe my argumentation and formulate the specific answer at the end.
Essentially, in the industry it is preferred to use twin engines vs quad engines based on the following arguments:

Twin engines have less maintenance cost (2 vs 4).
Less aerodynamic interference and weight.
Less cost (4 enginers are more expensive including systems for the same power)
However less bypass ratio for twin engines and overdimensioning due to compliance with the take-off certification at engine failure.

Netly, twin engines win against quad engines except in the past for ETOPS certification.
Ok, previous statement is my understanding of the trade-off between twin and quad. The article argues (using always seat mile costs) the following:

Little efficiency between B777-200ER and A340-300 / A380-800.
4 CFMs engines or A380-800 4 engines have less maintenance costs than B777-200ER 2 engines. Fleet of CFM is higher as well.
Argues that B777-200ER requires more powerfull engines. This argument does not contribute, as finally is an efficiency driver and the important is the first argument.
Previous comparisons are not valid to previous low bypass engines.
Article is not introducing airplane cost in the comparison (important for capital costs)

So, my explicit questions are the following:

Can somebody demonstrate with evidences that the efficiency of B777-300ER is lower or higher than A340-300 or A380-800?
Can somebody demonstrate with evidences that the maintenance cost of the engines of B777-300ER is lower or higher than A340-300 or A380-800?
Can somebody support further my arguments or support the article?

Kinda curious why the author compares the A340 to the 777 instead of the A330 which is essentially the same aircraft except for the number of engines. They were developed side by side and released within months of each other. If he wants to avoid apples vs oranges I couldn't think of a better choice

Yeah, and the A330/340 AIUI are more comparable to the 767, not the 777

Larger engine is more efficient in theory.  Four engines reduce wing weight.  There are many factors to account for but only two four engine planes for sale.  I don't know of a historical example of head to head same generation planes with same generation engines.  I believe the example in the article is for planes with some age difference.  A moot point for airlines as they will buy twins unless they buy the A380.

This seems more of a request to confirm your arguments, rather than a question about aviation per se.

@UnrecognizedFallingObject in the article he mentions that wants to avoid the technological effect. B767 is more comparable in terms of mission but has 10 years of gap

@TomMcW good point, however there is a significant range difference to be considered with detail in the comparison

@BrinnBelyea everything you mention is a design trade-off before selecting configuration. Finally implies efficiency (less weight, less comsuption), what finally really matters is the overall efficiency obtained. The article argues that the facts (without showing the data) show benefit over quads

@DavidRicherby no, in fact I want to know if the arguments of the article are really true.

This questions kind of feels broad, but for the A330/340 aircraft, I think the North American countries wanted a twin-engined aircraft, whereas Asian countries wanted four. Europe was split: you see airlines like Lufthansa and Air France with A340s, and KLM with A330s.

@BrinnBelyea: "I don't know of a historical example of head to head same generation planes with same generation engines" - A330/A340 (the A340 is an A330 with a four-engine wing).  "Four engines reduce wing weight" - not for the A340, which needed a heavier wing than the A330 in order to stiffen the wing and keep it from twisting dangerously under the weight of the outboard engines.

The main problem with the article as far as I can see is that they offer little in terms of evidence. The best we could do in the absence of their 'propriety model' is to use real world examples.

The best one is to use the Airbus experience in developing A330/340 which had little difference than twin/quad engines. While A330 is still in production, A340 has been stopped, indicating customer indifference. It is unlikely that the customers would've stopped operating A340 based on some myths.
Another thing to note is the present operation- Even in case of long-haul routes, where quads like A380 must be more efficient (in short routes, the more powerful engines of twins should help them reach cruise quicker), more twin engines are used compared to quads. In fact, the 777 LR is the predominant aircraft operated in ultra-long haul routes, followed by A380.
Also, look at the latest iteration of the 747, 747-8. Boeing is having trouble having takers for the jet (similar story in A380). In airline industry, where every single cent saved counts, almost all airlines are preferring twins over quads. As 747-8 has most of the aerodynamic improvements of current generation aircraft, maintenance should be an important factor in airline's decision in not selecting it.
Another important point- no quad is under development right now. With more and more aircraft development being done in collaboration with airlines, this is significant. in fact, since ETOPS came into force, quads are being slowly phased out (like trijets before).
The article never makes any point to support its assertion that the maintenance cost of twins and quads are the same. At the simplest level, assuming that they both have the same high-efficient engines, quads should take twice as the twins in man-hours for maintaining. For airlines, where engine maintenance takes a significant chunk of total maintenance, this is important. Also, more engines means more points of failure, which practically increases the inventory.
While one could argue that the quads can have better designs compared to twins (by virtue of improves wing bending relief, for example), this is questionable and the associated systems (fuel, fire suppression) would add more complexity and weight, compared to the twins.

Why would Airbus go to the effort of developing the A330 when they could offer the A340? Both have the same airframe and differ mainly in the number of engines. With ETOPS requirements at the time of development (when they were known as TA9 and TA11) being just 2 hours (ETOPS 180 operations were only begun in 1989), the only reason for developing the 4-engined variant was its capability of flying long overwater routes.
The time it takes to check an engine is the same regardless of size, so having only two of them saves time and money. In addition, the higher power reserves of a two-engined aircraft allow for higher climb speeds and enable it to reach the most efficient cruise altitude faster.
Today, most airliner engines are no longer owned by the airlines, but leased from the engine manufacturer. They offer a simple thrust per hour model where the airline knows exactly how much the engine will cost them over time, maintenance included. The only open issue is how long the airliner is grounded due to unforeseen maintenance, and with four engines per aircraft the number of unforeseen snafus is twice that of a twin.
At the same time, the airline is in the best position to judge how expensive a seat-mile in any of their aircraft is. Engine cost can be calculated very precisely, so every airline should have a very clear picture of how expensive their twins and four-engined aircraft are.
There is a small chance that the geared turbofans which are now being introduced will give the A340 an edge over a twin variant. The higher thrust class of large twins makes a geared fan currently impossible, so a four-engined aircraft will have an edge because it can use a more advanced engine.

Anyhow, the author argues that the maintenance cost is not highly different. The theory for the maintenance cost is clear, anyhow the article puts on the table (without showing data) that the factual situation does not show that difference.

@TrebiaProject.: You pointed it already out in the question, and I fully agree: Nowhere does the article show any trace of proof. It just claims it is so. Sounds like religion to me, not sound proof.

What I am looking for is somebody that, based on evidences in operation, that can support or challenge the article. For the moment I agree with you,  that article is based on faith.

The article is based upon a small sample size.  The price asked by an engine manufacturer is what the market will bear, not what it costs them to maintain the engines.  With such a small sample size there is no easy way to determine whether the prices are inherent or due to a markup.  Airbus and Rolls Royce offered a program to cut A340-500 and -600 engine costs to the level of the 777.  http://airinsight.com/2013/12/05/airbus-and-rolls-royce-announce-plans-to-restore-a340-economically-viability/

I've been thinking about the cost of maintenance and one explanation I can think for the assertion of the number of engines not making a difference in practice would be if the cost of maintenance per plane is dominated by the need to make sure enough planes are available even if some minor repairs are needed. The time needed for routine inspections and maintenance really might make no practical difference in that case, since it would be small in proportion to the time reserved in case issues exist. It would still reduce time available for other things, but the cost of that is not per plane.

Meaning there would be cost on the number of engines your fleet has in total, but not necessarily on how many a particular plane has. If this is true a simple comparison of maintenance cost between two and four engine planes would see no practical difference. You'd have to compare the effect of fleet composition to the total maintenance cost.

Thanks for the edit on maintenance. Is it really per thrust? Since if it is then four engines might get some benefit from that. I think planes are still supposed to not crash if one engine fails. With two engines that means installing twice the minimum needed. With four just 33%. No? Obviously two engines have less drag and moving parts to balance that. Also aren't engines changed before they start failing? So engine failures might be rare enough for a difference in engine number not to be significant. Sorry to bug you, but you seem to be good at answering questions, even from comments..

@VilleNiemi: It's still twice the minimum with quads, since they have to be able to fly with two engines out.  Trijets have it worst of all: they need to pack thrice the minimum thrust, since they need to be able to maintain flight on just one out of three engines.

@Sean: Proof, please! So far, all regulations I know just demand to cover one failed engine with full weight. Twins get some slack because they need to climb less than quads with one failed engine, that's all.


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Why don't fighter jets take off vertically?
I've heard the claim repeated many times that some modern fighter jets have an installed thrust greater than their weight, so theoretically they could accelerate straight up.
I've never actually heard of this being done, however. It sounds useful, though, especially on the deck of an aircraft carrier. Imagine: we could take out the whole catapult system, and replace it with some sort of a system to pick up an aircraft and turn it vertical.
Why is this not done?

Who says they don't?

@cpast good point, though VTOL aircraft are designed from the beginning for that capability and lose out significantly in other areas because of it. My proposal was more that a current F-16 with little modification could take off vertically.

The pick-up and turn system would probably not be that simpler. Some navies instead get rid of catapults by using the STOBAR "ski-jump" system. However advantage of catapult is that it can also launch lower-power planes like the E-2C Hawkeye.

And even VTOL-capable aircraft will avoid vertical take-off where possible.  AIUI even a short-take off run allows a significantly higher take-off weight which equals more fuel and/or weapons.

Also, doesn't the typical jet engine have to be moving at some speed in order to develop its full thrust?

As a side note, just because an aircraft isn't designed for VTOL, doesn't mean it can't accelerate straight up. Lots of fighters which take off normally can and due (in training, at least) accelerate straight up. They take off normally in order to get up to a controllable speed, then pull up. See here for example. Also here.

@NigelHarper: Remember that you don't need or want to carry more than a small amount of fuel on takeoff - you just need enough to make it to the tanker.

"Why don't fighter jets take off vertically?" and "so theoretically they could accelerate straight up ... Why is this not done?" are two completely separate questions.  Which are you really asking?

They can climb vertically, but this works best if they are several tons below their maximum take-off mass. Fighter jet engines need a lot of fuel, and at the beginning of the flight the aircraft will be too heavy for vertical climb. Also, the landing gear would need to be rearranged if the plane is to take off from any airport.
Even a thrust/weight ratio slightly above 1 at maximum take-off mass will not be enough, because the aircraft needs some airspeed for its control surfaces to become effective. If no thrust vectoring is installed, the aircraft will be uncontrollable in its initial ascent. The Harrier VTOL jet uses bleed air which is ducted to nozzles at the extreme ends of fuselage and wing for low-speed attitude control.
It is conceivable that the fighter will hang vertically on a wall, with its wheels locked in clutches which will release it when the needed thrust is reached. With thrust vector control the aircraft could be controlled over the full trajectory until it transits to horizontal flight, and could even land vertically. But this would need specially prepared airfields and use much more fuel than a conventional take-off, leaving less fuel for the mission.

Excellent answer, I think you hit all the points. The fact that carrying enough fuel to complete a mission adds to the take-off weight so much, and the uncontrolability are very good examples of why this is not done. Maybe with the advent of a more efficient thrust generating device this would be possible though :D

It is worth noting that there were several prototypes of Tail-sitters that were designed for such vertical take-off, but due none made it to production, presumably due to difficult handling and necessary design compromises.

The Convair Pogo was one such example.  I believe only the original test pilot ever attempted to land the aircraft, which required him to look behind himself to stabilize the thing.

@JanHudec I think that could make a good answer!

Peter, I think it might be best to take out the parts about specially prepared airfields. If this were useful, it would only be useful on a carrier, which could definitely be a specially prepared airfield. Otherwise, the answer looks great!

@raptortech97: Why restrict them to carriers? They could be on top of buildings, or in hardened silos. If no runway is needed, you can put airfields in a lot of places.

@PeterKämpf wow, that's an interesting idea!

@Peter So basically like helipads, but complicated and very expensive?

@cpast: The expensive part is the fighter. The airport is quite cheap; you need a wall and a restraint system, but can do without the full runway, and taxiways, too.

Am I right in thinking that most engine installations cannot generate maximum thrust at zero airspeed, primarily due to inefficiencies of the intake and a lack of pressure recovery, especially if they are designed for supersonic flight?

@sdenham: Absolutely right for fighter jet engines, but airliner engines have their thrust maximum at fairly low speed due to the high bypass ratio. That is the reason for using the big blower in the F-35 which is much better in converting engine torque into lift at low speed than the engine compressor and nozzle.

@PeterKämpf: "use much more fuel than a conventional take-off"
Can you elaborate about the fuel argument pls.
In order to gain potential energy (altitude), a certain amount of chemical energy (fuel) needs to be spent.
Wouldn't that be roughly the same amount when flying straight up compared to sloped? Couldn't it even be argued that straight up will be more fuel efficient, as less drag will be encountered during the shorter and slower vertical flight path?

@Scrontch: You need to support the mass of the aircraft during the ascent. Wings do this 10 to 60 times more efficiently than engines. The energy to lift and accelerate the aircraft is the same, so how you support the mass for those crucial minutes makes all the difference.

@PeterKämpf: I've often heard that argument but I still don't understand this. An engine burns fuel to accelerate air and thus get a force in reaction. How can it be more efficient to accelerate air backwards over a wing (that somehow deflects the airflow downward) than to accelerate it straight down right away?

@Scrontch: The secret is in the amount of air and the magnitude of acceleration. With lots of air but little acceleration, wings are more efficient that propellers, which are more efficient than turbofans, which are more efficient than turbojets.

Yes, they can accelerate straight up (even at max weight in some cases), but to accelerate straight up from 0 airspeed requires some kind of control to keep the aircraft stable.  All of the aircraft's normal control surfaces only work with air flowing across them, so if you stood it up and pushed the throttles forward it would simply tip itself over.
This is why VTOL aircraft always have more than one point creating thrust, they use thrust to stabilize the aircraft as it rises.
The other consideration (as Peter mentioned) is that it's less efficient to climb this way, meaning less fuel for the mission, lower takeoff weight, or some other tradeoff.

The closest the US ever got to it is the ZEL program for the F-100 Super Sabre.
Basically, put the F-100 on a cruise missile launch rail, strap one hell of a big rocket booster to its arse, and run for cover.
Used to have a 90 minute VHS tape about the project, they got so far as to design underground launch ramps with nuclear blast proof doors to launch the fighters after Soviet bombers. Never went operational though, by the time it had progressed to where it could work in practice the Nike SAM system was mature enough the ZEL program was no longer needed.

While the Super Sabre was certainly much earlier, it's not quite the closest the U.S. has to a VTOL fighter. There is this, after all.

@reirab the AV-8A was the earliest VTOL fighter the US entered into production.

Right. I just assumed you weren't counting that one since it was originally a British design and you said the closest the US got to a VTOL fighter was the ZEL program for the F-100. The F-35 is very much VTOL capable and is primarily a U.S. design.

Well they sometimes do.  A pilot or flight lead can request an unrestricted climb to cruising altitude for the purposes of practicing intercepts or other training maneuvers as this F-22 Raptor demonstrates.  But this does employ a conventional takeoff and accelerating the jet to a pre-determined speed before going vertical.

In regards to an engine thrust greater than 1:1, this applies to static conditions at sea level, generally using full afterburning.  As you increase in altitude, the rated thrust that the engines can produce degrades with decreasing air density.
The idea of a pure VTOL fighter has been explored with the "tail sitter" designs such as the Convair XFY-1 Pogo or Lockheed's entry.  These designs were feasible for a proposed cruiser based fighter interceptor, but the configuration made it difficult to land and the project was abandoned shortly after a series of test flights.
Both the Harrier series of attack aircraft and the new F-35B can take off and land vertically, but at a greatly reduced fuel an dpayload required for hovering; These aircraft prefer short take-off operations from ship with a warlord, then land vertically once this is depleted with completion of a mission.

One other factor to consider: if a fighter with a thrust to weight ratio of greater than one can launch vertically off of a ship, how does that fighter land back on the ship? Being out in the ocean, there's no where else to land, and if the ship has a flight deck for landing, may as well use it for a much safer conventional takeoff with a much higher combat load.
This was the issue faced with the three tail sitters the US financed in the 1950's, from Convair, Lockheed, and Ryan. The bottom line: landing on the tail, with the pilot looking over their shoulder in the attempt, turned out to be far too difficult, on stationary land, with a huge landing pad, and no battle damage. Trying to land a tail sitter on a moving ship after carrying out a combat mission would have been even more difficult.
All three efforts were abandoned as being impractical in military conditions.

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Does an elliptical wing planform have an elliptical lift distribution, and is that the most efficient shape?
I know that there are similar topics already here, but after reading them I still do not understand everything. From what I know an elliptical lift distribution gives the least amount of induced drag. That is because there is the least amount of lift at the wingtips compared to other parts of the span - is that true?
Does the elliptical planform generate the least amount of induced drag? I read somewhere that this wing planform is the most efficient planform in subsonic flight. Is that true and if yes why? And does elliptical wing have elliptical lift distribution? Because I heard lot of versions.

… elliptical lift distribution gives the least amount of induced drag.

Not quite. The elliptical lift distribution (read JZYL's answer for an explanation what it is) will only give the least amount of induced drag for a given span. That means that wings with more span and a more triangular lift distribution will be lighter, thus needing less lift and, consequently, producing even less induced drag than that elliptical wing. This was shown by Ludwig Prandtl in 1932 and R.T. Jones in 1950. For his elegant proof, Prandtl used the moment of inertia of the lift distribution which is smaller when more of the lift is concentrated near the wing root as a proxy for wing mass.

That is because there is the least amount of lift at the wingtips compared to other parts of the span

This is true for all realistic lift distributions. Even a triangular wing (one with zero tip chord) which has its highest lift coefficient near the tip will have its lift drop to zero at the tip. The reason for the low induced drag can be found in the downwash, which is constant over span for an elliptical lift distribution.

Is this planform generating the least amount of induced drag?

The least amount of induced drag is only possible with the least amount of lift. A more practical figure of merit is how much lift on top of what is needed to keep the wing aloft can be produced for the least amount of induced drag. This will be driven foremost by span: The more wingspan is available to produce a given amount of lift, the lower the induced drag will be. By using flaps or twist, the lift distribution can be optimized further, again (almost) independent of planform.

… this wing planform is most efficient among other planforms in subsonic flight.

No, and on top of that it has unpleasant stall characteristics. Next, scaling laws have also a say what planform gives best efficiency. Large aircraft are better off with a smaller taper ratio.

does elliptical wing has elliptical wing distribution? (I read this as lift distribution)

Yes, this is the one undisputed advantage of an unswept elliptical planform without washout: It will have an elliptical lift distribution over the full angle of attack range with fully attached airflow. As soon as separation starts, however, that is no longer true. But all other planforms need to be tweaked to produce an elliptical lift distribution and then do it only at one angle of attack.

Because I heard lot of versions.

Oh yes, there is a lot of mythology to be found on elliptical wings. Mostly because of oversimplification. I prefer to keep things as simple as possible, but no simpler. That is why this answer has become so long.

Prandtl ultimately showed that a span loading with negative lift at the tips was the lightest for a given amount of lift. This is now being revisited by the Prandtl-D project (see https://en.wikipedia.org/wiki/Prandtl-D). Since both weight and tip losses contribute to drag, the optimally efficient span loading will probably be a compromise between this and the elliptical form, but I do not know if it has yet been identified.

Thank you very much! Everything is clear now.

@GuyInchbald From Prandtls paper: Hier verliert aber unsere Aufgabe ihren vernünftigen Sinn, da hier negative Auftriebe an den Flügelspitzen auftreten und infolgedessen auch negative Biegemomente, und natürlich den negativen
Biegungsmomenten keine negativen Holmgewichte entsprechen, sondern wieder positive Holmgewichte. So he specifically excluded negative lift. I'd be interested in the paper where he included negative lift at the tips.

@PeterKämpf. You are correct. When I revisited the original studies for another thread, I realised that I was misreading one of the graphs: the tip lift was shown negative relative to a conventional wing. (Embarrassing, as I sometimes pick up on other misconceptions where relative and absolute values have been confused.)

As an OBTW, the min induced drag being elliptical is due to the Biot-Savart Law and the rule for how velocity decreases as you move out in radius from a vortex ($1/r$ for infinite vortex). If you changed that to ${1/r}^n$ from some $n \neq 1$ the wing would not be elliptical for min drag, but the downwash would still be constant. A small mathematical oddity.

Lift force over the wing is said to be of elliptical distribution if the lift per span ($L'$) along the wing span is of the following:
$$L'(y) = \rho_\infty V_\infty \Gamma_0 \sqrt{1-(\frac{2y}{b})^2}$$
where $\rho_\infty$ is density of the airflow, $V_\infty$ is the free-stream airspeed, $b$ is length of wing span, $y$ is lateral coordinate along the wing (0 being wing root), and $\Gamma_0$ is the circulation at wing root.
The above is essentially an equation of an ellipse, and hence the name elliptical lift distribution.
In low Mach flight, it can be theoretically demonstrated that an elliptical lift distribution produces the least induced drag for a given flat span. Induced drag of a lifting surface in incompressible flow can be expressed as (Ref Anderson, Fundamentals of Aerodynamics):
$$C_{D_i}=\frac{C_L}{\pi e A}$$
where $C_L$ is the lift coefficient, $A$ is aspect ratio and $e$ is the span efficiency factor.
For a flat span, $e$ must be smaller or equal to 1, and it's only equal to 1 when the lifting surface has an elliptical lift distribution.
When the wing has zero twist, an elliptical planform produces an elliptical lift distribution. However, elliptical lift distribution can also be coaxed by judiciously twisting the wing for a planform that is not geometrically elliptical.
Addendum on non-flat span:
In the case where the span is not flat, either because it has dihedral or is curved, it has been theoretically demonstrated that a non-flat span can achieve lower induced drag than a flat wing of equal projected span. This forms the theoretical basis behind winglets, end plates, spiroids, etc.
However, if we compare the non-flat wing to a flat wing whose length is that of the unfurled non-flat wing, the flat wing always has lower induced drag. That is, if there is no limit on the wing span length, elongating it is always better from an induced drag perspective.
Below is a graph comparing the induced drag of an optimal non-flat (circularly cambered wing) wing to that of an elliptically loaded flat wing. A value of 1 indicates no change. A value greater than 1 indicates more induced drag and a value less than 1 indicates less. $\beta$ is the span camber factor.

Data and figure compiled from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19630006412.pdf

Ok, so on elliptical planform the greatest amount of lift at the wing root and the least at wing tips? That is elliptical lift distribution?

@Konrad Yes as far as that goes. Elliptical describes more than just the lift at each end, it describes the shape of the whole graph of lift vs span.

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text:
Has the FAA banned FPV for model aviation?
On the FAA website the FAA says that you should not fly your model aircraft out of line of sight. Does this mean you cannot use FPV goggles?

The FAA has interpreted the rules to mean that the operator must have a line-of-sight view of the model aircraft at all times, and that FPV does not qualify.

Based on the plain language of the statute, the FAA interprets this
  requirement to mean that: (1) the aircraft must be visible at all
  times to the operator; (2) that the operator must use his or her own
  natural vision (which includes vision corrected by standard eyeglasses
  or contact lenses) to observe the aircraft; and (3) people other than
  the operator may not be used in lieu of the operator for maintaining
  visual line of sight.

But also note that this does not apply if:

...the aircraft is operated in accordance with a community-based set of
  safety guidelines and within the programming of a nationwide
  community-based organization;

The above is from the linked NPRM released in June 2014. The latest NPRM from February 2015 has the same requirements.
So members of an organization like AMA are exempt from that rule. The AMA requires a spotter to be used when flying FPV.

The current AMA FPV rules are also functionally similar to the FAA rules (since your spotter needs to be next to you and ready to assume Visual Line-Of-Sight control of the aircraft). So basically no cross-country FPV flights unless you and your spotter are being driven around in a truck following the model aircraft.

unless you and your spotter are being driven around in a truck following the model aircraft - There is actually an FAI competition class that is endorsed by the AMA for RC gliders called F3H (Cross Country Soaring) where pilots follow their gliders being driven around in a truck. So it's not completely unheard of.

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How many passengers can an Airbus A380 carry?
What is the passenger capacity of an A380, and how many suitcases could it carry?

Although it's fairly clear what you're asking, a little context around the question may improve it. Are you specifically asking about the maximum possible capacity, or are you asking about current real-world practices?

Assuming around 160t capacity for an empty A380 and an arbitrary average passenger weight of 85kg you could carry 1882 of these persons, it would just be a bit crowded. Assuming a suitcase weights 1kg you can fit 160000 into the plane, though I don't know if the density is big enough to fit in volume there.

747 record: "The operation set a world record for single-flight passenger load on May 24, 1991, when an El Al 747 carried 1,122 passengers to Israel".

I have voted for closure of this question as this is too broad, there are many variants of the A380 and hence the passenger carrying capacity will differ..

Wikipedia lists three basic configurations for the A380-800:

407-526 (3-class)
644 (2-class)
853 (1-class)

However, different airlines have different configurations. The lowest number of seats is Korean Air, with 407. The highest number of seats is Air France, with 538. Emirates has discussed plans for a configuration with 644 seats but it does not appear on the list (or on SeatGuru).
As far as baggage, this thread states that the A380-800 might carry 19 LD-3 containers of baggage, or 3002 cubic feet, which is for about 471 passengers. The bulk hold, which is for loose baggage, can hold 1,525 kg or 14.2 cubic metres (501.5 cubic feet), and the A380-800 can carry 8,000 kg of cargo in addition to a full passenger load according to Emirates.
An average suitcase is about 0.1 cubic meters in volume. If the bulk hold is 14.2 cubic meters, then it could theoretically fit 142 suitcases. But an average suitcase weighs 50 lb, so 142 suitcases would weigh 3220 kg, which is over twice the weight capacity.
The LD-3 containers add an additional 85 cubic meters of space, or 850 bags. This is a total of 942 bags, which for around 500 passengers sounds very reasonable.
But wait, there's more! What if the rest of the cargo capacity was filled with luggage?
Emirates above claims a total of 12 PMC and 44 AKE of cargo capacity. The properties are as follows:

PMC

23 cubic meters (max)
6700 kg capacity

AKE

4.3 cubic meters
1497 kg capacity

This is a total of 465.2 cubic meters of capacity, or 4652 suitcases, or 105,505 kg. The containers could hold a maximum of 146,268 kg.
So the bulk plus container capacity is 4794 bags by volume. This is assuming every container could be packed completely full of bags with a density of 227 kg/cu m, so the actual numbers would probably be lower.

Hmm, Wikipedia says 38 LD3s. Possibly some factor of 2 has gone astray somewhere.

@HenningMakholm That is just the luggage LD3's. Working on a total capacity now.

Note that the 50 lb is a normal maximum size for a check bag without paying extra overweight bag fees, not necessarily the actual average weight of a check bag. Good answer, though. In Korean Air's case, their smaller number of seats is due to devoting the entire upper deck to business class and having the first-class cabin taking up the front of the lower deck as well as having a couple of lounges and an on-board shop. They also use 33-34" seat pitch in economy instead of the 32" pitch Air France uses.

@fooot so 19 LD3s stands?

@vasin1987 for the typical number of containers needed for luggage, yes.

In theory, up to a maximum 853 passengers in a single-class setup (ie if the whole plane was set up for the maximum possible number of economy passengers)
In practice, it's usually configured for 525 in a typical "three class" (First, Business, Economy) layout, which is standard (with fairly minor variations) for most airlines as this is both more profitable to the airline, and better suited to actual real-world demand.
For example the upper deck contains a small number of First Class seats (front) and a larger number of Business Class (rear)

While the lower deck contains all economy class passengers

Luggage isn't a limiting factor for the A380, as it can hold approximately one suitcase per passenger in the luggage hold, and a further 2-4 suitcases per passenger in the bulk hold (depending on whether other cargo is being carried)
Reference: airbus.com
Airbus has some vague plans to introduce a 1000-passenger all-economy (ie single class) "stretched" version of the A380, but these don't appear to be in the pipeline for production anytime soon. The A380 was originally designed with this in mind, though.

The Daily Mail and Seat Guru both show Emirates having a two class version with 615 seats total, 557 economy. And, while Emirates only shows three class versions if you actually look up the route mentioned by the first link (Dubai - Copenhagen) you will see it only has Business, Economy. Clicking on the "View services" link will also show the First class missing. So, despite the Daily Mail being a rag and Seatguru not being 100% reliable to say the least I must say the evidence strong for this configuration.


text:
What categories/classes is a Ground Instructor able to instruct on?
Under §61.215 Ground instructor privileges, it says

(a) A person who holds a basic ground instructor rating is authorized
  to provide—
(1) Ground training in the aeronautical knowledge areas required for
  the issuance of a sport pilot certificate, recreational pilot
  certificate, private pilot certificate, or associated ratings under
  this part;
(2) Ground training required for a sport pilot, recreational pilot,
  and private pilot flight review; and
(3) A recommendation for a knowledge test required for the issuance of
  a sport pilot certificate, recreational pilot certificate, or private
  pilot certificate under this part.

But it doesn't say anything about what kind of category and class the ground instructor may instruct on. Is a separate GI rating required for each category and class? or is there one GI rating that covers all categories and classes?

The basic ground instructor covers everything you quote.  The only additional privileges you can get are those granted by the advanced ground instructor rating.  My ground instructor certificate just says "Ground instructor" on the front and "advanced instrument" on the back.  No other ratings or limitations.
Things like reading charts, familiarity with regulations, weather information, fundamentals of flight and other required knowledge topics do not depend on category or class of aircraft.  You just need the BGI (and IGI for instrument) to teach ground and endorse written tests regardless of the aircraft the student is or will eventually train in.
The only other benefit to having a ground instructor certificate is that it is required to get a gold seal on your CFI certificate.

The AGI does not allow you to endorse instrument tests.   The IGI allows that.

@wbeard52 edited. thanks

There are three different types of ground instructor ratings.

Basic - allows you to provide ground instruction to sport, recreational and private pilots.  You can even provide the ground portion for a flight review.
Advanced - allows you to provide ground instruction for sport, recreational, private, commercial and airline transport pilots
Instrument - allows you to provide ground instruction for an instrument rating.

The test bank includes CFI questions from all category and class aircraft so you are authorized to provide the ground school for everything from balloons to gliders to helicopters to airplanes and everything in between.
AC 61-65 clarifies the requirements for a gold seal.  If you are a sport pilot instructor a gold seal can be added with a basic ground instructor certificate.  For other CFI's (not sport instructors) an advanced or instrument ground instructor certificate is required


text:
Why do some airplanes have vertical strakes?
A lot of different aircraft have vertical or near-vertical fins below the empennage, or above the empennage and attached to the front of the vertical stabilizer.
What purpose do these devices serve?

Strakes serve to improve lateral stability in airplanes which may have certain features that make them less stable in yaw.
For example, strakes are especially popular on floatplanes, and, indeed, the Floatplane Flying Handbook says:

On many airplanes, directional stability is affected to some extent
  by the installation of floats. This is caused by the length of the
  floats and the location of their vertical surface area in relation to
  the airplane’s CG. Because the floats present such a large vertical
  area ahead of the CG, they may tend to increase any yaw or sideslip.
  To help restore directional stability, an auxiliary fin is often added
  to the tail. Less aileron pressure is needed to hold the seaplane in a
  slip. Holding some rudder pressure may be required to maintain
  coordination in turns, since the cables and springs for the water
  rudders may tend to prevent the air rudder from streamlining in a
  turn.

There are also STC kits that add strakes to different airplane, and whose manufacturers claim a variety of improved handling benefits, including "Increased Yaw Stability."

The vertical tail surface is like a wing, maybe with a smaller aspect ratio, but it works much the same over sideslip as a wing works over angle of attack. The fuselage, however, is a slender body with a much smaller lift curve slope, but a much higher "stall" angle. In fact, its lift curve slope is nonlinear and tends to increase with higher angles of attack and angles of sideslip. If you want to know more how the lift curve slope varies with aspect ratio, please read this.
This means that at some sideslip (around 15° for most configurations) the vertical tail stalls and its stabilizing side force decreases with further increase in the sideslip angle, whereas the destabilizing influence of the fuselage increases further. The aircraft becomes laterally unstable pretty soon.
To buy a little more stability around the point where the vertical tail stalls, these strakes have been added. They have a high leading edge sweep, so they will produce vortex lift just like a highly swept delta wing, only sideways. The suction force of this vortex will act on the root of the vertical, just were it has the biggest chord. In fact, this helps to keep most of the vertical active, because now it becomes part of that delta wing ahead of it.
The strakes in the upper picture have a different purpose on top, however. They limit flow separation on the rear fuselage in high speed flight by preventing crossflow. Actually, they are an embarrassment for the aerodynamicist, because they show that the lower fuselage suffers from flow separation just when it hurts most.
Most military cargo aircraft have them, because in order to keep the ramp short and to provide enough clearance for rotation, their tailcones are steeper than what the airflow can manage. Among modern military transporters, only the Lockheed C-5 (thanks to @dotancohen for the hint!) and the Antonov-70 can do without them. This was a definite source of pride for the chief aerodynamicist of the Antonov Design Bureau, Oleg Bogdanov.
The lower fin in the second picture was added to compensate the destabilizing influence of the two floats, which will destabilize the aircraft in much the same way as the fuselage does.
Extending the vertical downwards also helps to improve handling qualities and should be done as much as tail strike clearance allows.

I think that the C-5 Galaxy is missing the strakes as well, unless I misunderstood you.

@dotancohen: I never noticed, but you are absolutely right!


text:
How do jet fighters get to war?
Currently Australia is deploying F/A-18E/F Super Hornets to the UAE to fight in Iraq. Obviously the UAE (and Iraq) is a very long way from Australia and well beyond the typical range of one of these aircraft.
On TV they literally showed the Australian Prime Minister waving goodbye while the jets took off and flew into the distance, which for some reason seemed odd and made me wonder about how fighter jets are typically ferried to a distant war.
I kind of assumed they were loaded onto an aircraft carrier and shipped there, but Australia doesn't have an aircraft carrier so obviously this isn't the case.
I assume they must therefore fly, but:

Do they fly hop to hop within their usual range refuelling at each stop? Where would these stops be? I assume they can't just refuel at a civilian airport in an arbitrary country and would have to be an air force base belonging to a close ally.
Do they fly continuously with aerial refuelling? Does the tanker need to make hops to refuel?
Do they swap their weapons for extended fuel tanks which provide enough range to fly continuously to the UAE?
Are they actually shipped via regular military cargo ship and the "departure" on TV just for show?
If they do fly continuously, it seems like flying non-stop for ~13 hours in a cramped fighter pilot cockpit would be unreasonable (even commercial pilots take shifts), or would they fly supersonic and perhaps reduce this to 5 or 6 hours?

Answers don't necessarily have to be Australia -> UAE specific, I'd also be interested to know how the US deployed to Iraq and Afghanistan for example and if the process is consistent or depends on the aircraft and location.

To fly the planes across half the globe is easy. The hard part is to ferry all the rest there in time, so operations can start soon. You wouldn't believe how many tons of equipment and how many people are needed to keep one modern fighter aircraft flying.

Exactly, flying the fighters there is easy, the real question is how do you get the bombs, replacement parts, and service personnel there.

Another (much more expensive and only used as a last resort) option is to put them in a C-5 Galaxy. In a situation where an F-18 was damaged to the point of being unflyable on its own, the USAF took its wings and stabilizer fins off and flew it back to the U.S. in a C-5.

Regarding the fourth hypothesis: Military equipment (not specifically fighter jets) is sometimes shiped through commercial cargo ships, even for the US.

All correct, but the support isn't needed immediately and it doesn't have to come from the same place. In this example, Australia is sending F-18s to the Middle East. The USAF is already there, is somewhat familiar with the F-18 and can handle whatever is needed until the AU support staff arrive by ship.

@paul, support staff are easy, a couple of C130 loads or a couple of chartered airliner or two. Although you can fly engines, spare parts, and even weapons, it's much more economical to put them on a ship.

I'd just like to point out that F-18E's have much larger fuel tanks than the C versions, and the E can carry 3 large external fuel tanks that more than double its fuel capacity iirc. Plus, it is capable of air refueling. And the hornet is far from slow.

Dear any fighter jet pilots looking for answers on this question: you looked in the wrong place.

Regarding your final bullet point, demanding people do unreasonable things is traditional for the military, and I have it on good authority that fighter pilots hate transporting planes.

To answer the list in order:
Do they fly hop to hop?
An F-18 can go 3,000km unloaded (ordnance adds a lot of drag) and that's plenty from Australia. You can get to the Middle East in 4 hops, no air-to-air refuelling required. Although if you do an in-flight top-up you can add more than the regular range because you don't need to burn any fuel getting back up to cruise altitude.
Where would these stops be? Surely not at civilian airports?
Military aircraft often use civilian airports - they run on Jet-A just like everything else. Fighters were not an uncommon sight at the general-aviation terminal where I'm from. Loaded warplanes are a political issue, unloaded ones are a good day's business for the airport's fuel merchants. (note that fighters are almost never completely unloaded - the ammunition for the gun has a significant effect on the airplane's trim. But you can't see it.)
An external fuel tank can be added in place of weapons.
Are they actually shipped via regular military cargo ship?
Fixed-wing aircraft are typically not boxed for surface shipping. Helicopters are - no wings to remove and rotors can be folded up very easily.
Carrier-based aircraft will depart from land - the only way you get a Hornet off of a docked carrier is with a crane, so they typically launch everything when still offshore, and load the aircraft the same way on departure.
The USA has enough carriers that they would not deploy a carrier-based fleet like this. If they are adding / replacing aircraft there are probably enough already in Europe - send those where needed and replace them in the normal course of events.
Would they fly supersonic to reduce travel time?
No, they won't go supersonic to reduce the flight time. > Mach 1 consumes so much fuel you'd never get there. Fighters have supersonic capability so they can get there or run away as needed, but it's never used for very long. The Concorde was designed from the beginning for Mach 2 cruise, so it didn't use afterburners once at Mach 2. Subsonic flight suffered as a result, fighters would not accept the subsonic flight handling issues as that's rather important to them :-)
Bombers are often deployed directly from home bases. All modern strategic bombers have global range with in-flight refuelling. You don't leave North Dakota, blow up something in Longwayfromherestan and get home for dinner, but you don't land anywhere else either. Yes, it's many hours in the same seat but the military crews are used to it.

Given all this info, my guess would be that OP's TV takeoffs were heading to meet up with their carrier offshore. Also, in terms of "butt in seat" time for a bomber on a long haul - those cockpits are probably much confier than tiny fighters would be.

@abbyhairboat except australia has no carriers for conventional fixed wing aircraft (harriers, yes).

"bomber...those cockpits are probably much confier than tiny fighters would be" - Bomber crews are strapped into ejection seats too. But they can get up.

@flyingfisch just a turn of phrase- I meant that the matter of what the fighters were doing taking off on TV is more complicated than I originally thought.

Some fighters (such as the F-22) have supercruise capability, so no afterburners required. No idea how it affects fuel consumption though.

Aren't the Aussie F/A-18s equipped for In-Flight Refueling? In which case they'd take off from a runway, stay at altitude for a LONG time, hit a tanker or two along the way and finally land at the destination.  When my (USAF) unit deployed F-16s to the Sandbox, that's what we did.  A bunch of Vipers and a KC-10 full of fuel, personnel and equipment.  The autopilot works well enough that the pilots don't get too fatigued (energy drinks help).  May have to use a "piddle pack," though.

@abbyhairboat Given all the info in the answer (in particular, the fact that a Hornet can fly for 3000km on one tank of fuel), I concluded that the televised take-off was the start of a four-hop flight to the Middle East. The answer almost says that in as many words!

Can a F-18 take off with ALL there tanks filled up?   If not mid-air refueling very close to take off location could make a big difference.

Military aircraft don't run on Jet A except in the United States; in most of the rest of the world, they run on Jet A-1 (like civilian jets and turboprops in said most of rest of world do too), while, in China and the former Soviet Union, they run on the Chinese and Soviet national jet fuels, respectively.

I am former fighter pilot:
We often fly "hop to hop" but depending on the total distance, the need for urgency, and the distance over water we also fly with tankers and air refuel. Of course the tankers sometimes don't have the fuel for themselves and for us. In those cases, we'll just leave one tanker and rendezvous with another tanker enroute. Everything is very carefully planned. The longest I spent in an F-16 seat was 14 hours. It wasn't too bad.

It's funny. Flying an F-16 has a glamorous and dangerous image. 14 hours alone with nothing to look at but a clear sky sounds... kind of boring.

@Chris, what altitude and airspeed do you maintain during long flights? Is it in the order of FL500+?

@Chris Do you guys have to plan alternate airports too for these logistic placement flights? It'd be mighty uncomfy to ditch a multimillion jet because your target hop airport shut down.

@curious_cat I think pretty much all pilots try to make sure they have alternate landing possibilities within their fuel range, especially for long distance flights.

@steveverrill except you have what, better than 180º of view around you. That's a heck of a lot cooler than the teeny windows you get on your run of the mill airliner...

@steveverrill - as they say, "War is weeks of interminable boredom punctuated by moments of sheer terror". Peace is the same, but more... :-)

How did you stave off boredom in an F-15 for 14 hours?

@LevelRiverSt spoke to an RCAF Hornet pilot a few months ago. Many of them carry an iPod with some audiobooks and music during long patrols or ferry flights.

@jwenting Does the Hornet really have an iPod jack?

@DJClayworth I guess they plug in the 3.5mm audio plug into their headset somehow, or wear earbuds under the headset. They didn't tell me how they did it, just that they did it.

Ferry range of an FA18-E/F (with ferry tanks attached) is ~ 1800nm.
There are any number of possible routes from YAMB to DHF. The choice of route will be influenced by weather, speed, and  politics (third country over flight and landing rights).  It is also possible to mix ferry and refueling.
One could, for example, do YAMB-DCN-CCK-NKW-DHF (see image below) with a mid-air refueling between NKW & DHF if one wanted, for whatever reason, to avoid landing in India or Sri Lanka.

Good Question! They are typically flown in legs from Australia, or if long distance over water, conduct air - air refuelling.
The Super Hornets you are talking about will most likely stop at somewhere like Butterworth in Malaysia, then fly onto Dubai, possibly meeting with the MRTT sent the day before to refuel enroute.
Australia does not have any Aircraft Carriers so sadly, we have to ferry flight them everywhere.

They may do hop to hop or they may use inflight refueling.
Of course that opens up the problem of the tankers having enough fuel. Not all tankers can re-fuel in the air, but some can.
When the RAF did the Black Buck raids in the Falklands war (~12000km), they were offered the use of US Stratotankers. They decided it would not work because the Stratotankers cold not refuel in the air (perhaps they can now, I do not know).
They used their own Victors, which refueled each other, ultimately leaving the Vulcan to carry on on its own.
Alternatively, you could use tankers that are already positioned on route, which was the USAF does when it B-52 strike the middle east from their bases in continental US

Whats the point of having a tanker that cannot refuel midair. It is the fact that they use different refueling system and that makes one useless tanker.

The really interesting story was the initial Vulcan raid on the Port Stanley airfield. One Vulcan, twelve Victors to refuel the Vulcan and each other, and they had only a couple of weeks to practice Vulcan aerial refueling, which they hadn't done in ages. They pulled it off. Good doc on this on youtube.

@tj1000 they also had to re-add the capability to the Vulcans too. Part of the re-fueling probe was being used as an ash tray in the mess at a nearby airfield apparently

You can't land just any plane on an aircraft carrier, it has to have tail hooks and the pilot needs special training. The Aussie F/A-18s are not equipped to land on aircraft carriers. So they have to do short 'hops' or in flight re-fuel.

The bigger issue here is that Australia doesn't have any aircraft carriers to land on.

@raptortech97 I am surprised that Australia does not have aircraft carriers, any reason

@Firee http://en.wikipedia.org/wiki/HMAS_Melbourne_(R21)#HMS_Invincible ... Canada doesn't either, nor Japan, nor any country in the Middle East nor Africa ... only about 15 countries do.

Actually, Japan sort of has carriers, they just don't call them that. They're called the Hyuga class 'helicopter destroyers', although an F35B could also operate from them. I'm sure that never occurred to the IJN when they built those 'helicopter destroyers'...

@ChrisW you'd think that a big island nation would want to have a navy to defend it's SLOCs.

@RonJohn The USA's military expenditure is nearly 30 times the size of Australia's. https://en.wikipedia.org/wiki/List_of_countries_by_military_expenditures

@ChrisW so... like Europe, Australia is riding free (not totally, but close enough) off the USA?

@RonJohn The States used to spend as much as the rest of the world put together! Now it's only 40% of world spending but no other single country is in the same league. Many countries spend 2% of their GDP -- that's the NATO target and Australia does that too -- America spends 4% of its GDP, but its GDP is so much higher too. Europe doesn't exactly ride free, its military spending is 50% of the States' (i.e. for the whole of Europe aggregated) but no other country has a navy to touch America's. FWIW America like other countries benefits economically from international trade (i.e. your "SLOCs").

@ChrisW Europe rides free, because much of what they call "defense" spending is really on internal security.

These articles [1, 2] from The Register describe the procedure followed, when a couple of F-35 jet were delivered from the US to the UK. Crossing the Atlantic in a peace-time delivery could well be compared with moving fighters from their home base towards a base nearer to the theather of conflict.
In one of the articles several means of delivery are discussed: direct crossing, fly a more overland route (as in pre ETOPS times), or delivery by ship.
Direct flight
With a direct long-haul flight, you need to take air refueling into account and take precautions for the case when something goes wrong. Especially, with single-engine planes. Thus, the F-35s were accompanied by tankers and a A-400 M to provide sea rescue capabilities.
Overland route
An overland route is the safe option for non-ETOPS planes. However, this involves a lot of diplomatic activity. In contrast to civil aviation, armed forces are supposed to ask prior to entering other countries' air space.
Delivery by ship
This is by far the safest option. However, this involves some logistics. First, (partial) disassembly, shipping and re-assembly at their destination. While being very safe, this is the least glamorous way to Air Force business.

Even being carried by ship carries some risks. This brand-new railway locomotive was damaged beyond economic repair when it was dropped while being unloaded.

Generally fighters will fly long ferry routes together as a squadron with multiple tanker rendezvous enroute for fuel.  Most of the squadron personnel and equipment will be airlifted in cargo aircraft to the base of operations but the fighters will be flown there by the pilots.  Flight planning generally dictates alternate land bases must be within flying range as well in the event an aircraft in the flight cannot successfully tank.  Cruise speeds for ferry flights are not supersonic but in the Mach 0.8-0.9 range.

Generally, ferry flights are done with aux tanks when possible, and no ordnance on the aircraft.  The logistics are influenced by the urgency, the geo political situation, the location of friendly countries, their assets, and airports to land at, and staffing.
Ferry flights are preferred over boats for many aircraft, as once an aircraft is on a boat is is typically not available for a period of time, and needs special equipment to load and off load.
Logistics has to handle overflight permits, etc. and rapidly deployed aircraft will often have over water routing to reduce permit requirements.  Additionally there may be some interest in reduced media coverage. Landing 16 fighters in Dubai to top the tanks is not done except where the visibility is desired, and most stops are done at bases or bases of allied forces.
It is noteworthy that certain flights, for example, B-2 runs, are originated out of mid-continent US, and return to that base afterwards. They are supported by tankers, and in the case of the B-2 it avoids the risk of landing a plane at a remote location.
For the US and allied forces, there is coordination by logistics, so that Navy assets are tasked with providing support in the unlikely event of off shore crew recovery, support, etc.  For example, I know that US Navy has been advised of AUS deployments, and provides offshore services for asset movements.
Such movements are routine.  For example, a NY Guard unit has supported South Pole NSF operations, and the C-130 used is ferried from New York to the South Pole and back after the missions are complete.

authors: Bob Jarvis - Слава Україні


text:
What is the benefit of a curved up flight deck?
A Russian aircraft carrier Admiral Kuznetsov has an interesting aspect: the edge of the runway is raised, which “pushes” the plane in the air during the takeoff. Looking at takeoff videos, the curved edge seems very natural.

All USA aircraft carriers I can think of have an absolutely flat deck, similarly to, for instance, USS Abraham Lincoln:

The Admiral Kuznetsov's curved deck provides benefits, so why were most carriers not built the same way?

Guys, please read the whole question before closing as a duplicate... the first part of the question, the benefits of the curved deck, is answered in the "duplicate" question, but the second piece of the question, why aren't most carriers built this way, is NOT answered in the "duplicate" question.  There is a valid question here which is NOT answered yet. Voting to re-open.

It's definitely an effect of thermal expansion, they used different material layers for the deck and they are above an underwater geyser. The effect will disappear with next shower. Complete demonstration on this scale model.

Are you serious? No this is a structural design meant to serve as a ramp, not thermal expansion lmao

To mention another disadvantage: if take-off doesn't quite work, you end up getting run over by the ship, which can make for a bad day.

"What is the benefit of a curved up flight deck?"  -- Up is always the best way to curve the flight deck.  Sideways or down don't work so well.

Because the method of launching aircraft are completely different in these carriers. Carriers with ski-jump, initially designed for RN Harriers (as @Simon already has explained), requires that the aircraft take-off under its own power.
On the other hand, large aircraft carriers having flat decks have their aircraft launched by catapults (with a notable exception) which basically throw the aircraft from the deck. Though this system is heavy, expensive and complicated, it brings significant advantages to the table:

It allows aircraft with higher t/o weight to operate from the carriers. USN operates F/A-18E/F Super Hornets, which weigh ~30 tons from its carriers (they even operated the heavier F-14 Tomcats). The ski jumps, on the other hand restrict the aircraft t/o weight (which means less ordnance), as the Chinese are finding out. Indian Navy operates Mig-29Ks from its carrier, but they are lighter (~25t).
The catapult launch offers more options - it can operate in wider range of sea conditions compared to the ski-jump and can even launch aircraft even if one of the engines fail during the launch, a feat usually not possible for the aircraft in ski-jump.
The catapult allows for a significantly higher takeoff rate, which means a higher sortie generation. As rusnavy.com notes:

With its four catapults, a US carrier is capable to shoot one aircraft in every 15 seconds. Kuznetsov has only three takeoff tracks. Moreover, two bow tracks are not intended for fully-loaded aircrafts! They can take off Kuznetsov only from one track which starts far behind the midship, i.e. an aircraft must run almost the whole flight deck! Comparing to catapult, takeoff rate of ski-ramp is at least twice lower.

The catapult allows the launching of support aircraft (like AWACS), which have low thrust-weight ratio and would not be able to take off under their own power.
The ski jump takes up space in two ways- physically, it occupies front deck space (on the other hand, aircraft can be parked over catapults when not in use), while the take off run required by the aircraft (greater with more gross weight) eats up more space, leading to limitations of the number of aircraft on the deck.

As for the exception, the USN operates carriers without ski-jumps, but these are not called as such- they are called amphibious warfare ships an operate only VTOL aircraft like the F-35 Lightening II and V-22 Osprey.

It's not that much the t/o weight but rather the thrust-to-weight ratio that plays the role here. (It's just that, very generally, larger aircraft have such ratio lower). Kuznetsov can operate Su-33, which is heavier than F/A-18F. But it couldn't, say, a lighter EA-6B.

This answer seems to mainly list the benefits of having a flat deck, but the question was about the benefits of having a curved deck

Actually, the AWACS can take off under its own power using the landing area as a runway.

@J.Hougaard The question asks "Why were most carriers not built [with ski-jumps]?", so this answer seems entirely on-point to me.

The ramp design may be referred to as STOBAR (Short Take-Off But Arrested Recovery), and this is a fairly simple design to implement. However, the complexity is pushed into the aircraft. Since there is no catapult assistance, aircraft have a hard requirement to take off in the space available, given the ramp at the end. This limits the types of aircraft that can be launched, typically to fighter aircraft that already have high thrust to weight ratios, or aircraft specifically designed for short takeoff capability. Takeoff weight may be limited. Aircraft still must be equipped with an arresting hook for landing.
A catapult system may be referred to as CATOBAR (Catapult Assisted Take-Off But Arrested Recovery), and this allows for launching a much wider range of aircraft. For example, CATOBAR carriers can launch large aircraft like the C-2 or E-2. These aircraft would not be able to take off from a carrier without catapult assistance, and provide carrier resupply or AWACS capability. Even fighters that may be able to take off without assistance can be launched with much more payload. However, this capability comes with a cost. The aircraft will need to have strengthened landing gear to withstand catapult launches. The carrier is also made more complex, placing an additional demand on a boiler or generator to provide the large amount of steam or electricity needed to launch aircraft. Modern US carriers are nuclear powered, providing the energy needed for this system.
There are also practical considerations in aircraft carrier design. The ramped deck may detract from available space on deck for parking aircraft. You can find many pictures of CATOBAR carriers with aircraft parked on the bow area, which would be difficult on a STOBAR carrier. The angled deck on modern carriers would still allow for launch and recovery of aircraft on a CATOBAR carrier with the bow area occupied. The ramp also reduces forward visibility for maneuvering the carrier. On a CATOBAR carrier, the flight deck must be designed to include the catapults, while on a STOBAR carrier, only the blast deflectors must be integrated into the deck.

Think of the ramp as free lift with no maximum lift coefficient so you can't stall while using it, and zero induced drag.
If you could make a catapult that curved upward...


text:
When is runway slope most important?
Based on this other answer, a new question arose.
When is runway slope most important, at take-off or at landing? Or is it equally important in both flight phases?

OP, I will not vote to close, but for the future, this question matches more than one shown on the [help] pages as 'what not to ask' StackExchange is not a place to argue/convince/discuss. Ask a question and reader vote on whether they agree or not. We do not, should not, invite discussion a la forums.

It's most important when you're coming up on the far end of the runway.

I would argue that it is most important on takeoff. The reason is that most planes require more distance for takeoff than for landing (partially due to higher weight). Also, it is generally easier to abort a takeoff than a landing (when on the ground at least).
Both landing and takeoff distance calculations are performed (which should take slope into account) to ensure that there is sufficient distance for the aircraft. However, in addition to aborting a takeoff being easier, more can go wrong in a landing. Windshear or improper approach settings can cause an airplane to touch down later or faster, which risks an overrun. During takeoff, the plane has until V1 (takeoff decision speed) to abort if any issues arise.
However, this certainly depends on conditions. Aircraft condition, wind and runway surface condition can increase the distance needed for landing (or aborting a takeoff). If reverse thrust is not available, there is a tailwind, and the runway is wet, that could change the situation.

I would rather land downhill than takeoff uphill because taking off uphill you have just one thing to get you off the ground: thrust (ok, maybe a little flaps helps in some airplanes). On landing, I have a lot of things I can control: approach speed (slow), approach angle (steep), approach power (none), flaps (full), flaring before the threshold, aerodynamic braking with the elevator, pulling the flaps, full braking. some of it is admittedly pretty gutsy, but you can stop even a slippery airplane quite quickly.

I am going to introduce another perspective. We are talking about a road of 3km length with a potential difference of 1% (30m!!!!) between the highest and lowest place.
Runaways are always thought to have enough visibility for the pilot when trying to land of take-off. Limitations of the slope in airports are not related to performance of the airplane, instead is a safety and visibility constrain.
So, either I am taking off or I am landing I prefer in both to be running uphill where my airplane at the fastest speeds, so I will be able to see any object really far from where I am. No matter the performance I lost.
Notice, that we need to consider also the visual range of the pilot (which is several meters above the road) which is not 3km for small objects, but, I prefer the safest condition: Uphill.

The only time you would worry about runway slope is insufficient runway. So let's consider a runway overrun:
On takeoff, there's a period where you should no longer abort takeoff, as there is likely insufficient room for stopping. That happens as the plane accelerates pass V1. At that point the aircraft is quite some way down the runway travelling fast. It's heavy as well. If a late rejected take off is initiated, you'll likely hit the fence at a very high speed.
On landing, you plan to stop the plane anyway. If things go awry, you can always step on the brakes. Landing distances are never computed at max braking so you got some room. If you overrun, you'll hit at a much slower speed, since the plane is slowing down in the first place.
Therefore I'd argue it is more important on takeoff.

For airline ops the runway slope is taken into account for the performance charts for that specific runway, so when calculating V1, V2, stopping distances, etc, this is all taken into account.  Also note that some airplanes have a maximum runway slope limitation that must be considered.


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Cabin crew : how do they get Oxygen in the event of decompression?
According to this answer, in the event of an explosive decompression it can take as little as 9 seconds to develop problems without an Oxygen supply.
How are cabin crew meant to get to an oxygen supply when they're a long way from their allocated seat ? There's not only distance to consider but if they're jammed between two carts in the middle of meal service they're not going anywhere too quickly !

There are usually extra oxygen masks in the cabin- it is a regulatory requirement. From 14 CFR 25.1447 - Equipment standards for oxygen dispensing units:

(c) If certification for operation above 25,000 feet is requested, there must be oxygen dispensing equipment meeting the following requirements:
(1) ... There must be ... at least two oxygen dispensing units connected to oxygen terminals in each lavatory. The total number of dispensing units and outlets in the cabin must exceed the number of seats by at least 10 percent. The extra units must be as uniformly distributed throughout the cabin as practicable.

So the cabin crew can use the nearest unused oxygen masks. Also, each flight attendant position has a portable oxygen system, which can be used in case of emergency (though it is intended usually for therapeutic purposes). in addition, there is also the Protective Breathing Equipment in the cabin. Though these two are not intended for normal use during decompression, I guess it can be used in case of an emergency.

Are you sure the portable oxygen is at each FA station? In the Helios 522 accident it seems the FA took rather long going from one of the extra masks to the next before he got to the portable one (and thus got to the cockpit too late to do anything). Or perhaps more of them were added in the wake of that accident?


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Help Identifying a LEGO plane
I need some help identifying a LEGO plane.
The LEGO set 7292, Propeller Adventures, features a twin engine airplane, with conventional landing gear, and a bubble canopy.  There are no pontoons on the wings
Assuming it's based on a real plane, does anyone have any ideas?

Related: On what plane is 7292 Propellor Adventures based?

Being a Lego fan for over 30 years, I highly doubt that it is based on any real life aircraft. Lego doesn't hide if a model is based after something from real life (examples).

The tail surfaces are far too small for any real airplane.

I'd like to know how this question doesn't appear to be about aviation.  It's about identifying an aircraft.  I mean, there's an entire tag about identifying aircraft.  The ideal place to ask about something like this is here, in a community of aircraft and aviation experts, not a community of toy enthusiasts.

@Nick2253 It has been put on hold because the model is a toy, not an actual airplane. The tag you mentioned is primarily used for pictures of airplanes.

In 8 months, about 100 views and no answer on the lego site, in 5 days, slightly more than 100 views but many suggestions on the aviation site. Seems to indicate that it really is an aviation question…

@Relaxed You can certainly vote to reopen it.

@Farhan I don't think I can, no (low rep).

@Relaxed Well you are not far from that.

@Relaxed I agree with you, and I voted to re-open.  I will admit that it is a bit of an edge case, but, I do think it is an interesting aviation related question.  One that this community could be very helpful in answering (as has proven to be the case, 2 answers already) and one that people find interesting (760 visits so far...)  I would like to see more evidence behind why it would be a particular craft (rather than just pure speculation), but I still find it interesting...  So I've nominated it for reopening.

There are a few which are similar on some points, although all have at least one issue. In the end I suspect in the end it's just a "generic mid-wing, twin engined WWII fast bomber"
Other than the radial engines, it looks a lot like a de Havilland Mosquito

Or perhaps a Junkers Ju88

Or a CAC Woomera, at a stretch

Could also be a Ki-45

or except for the tail a BR-20

It doesn't quite fit, to my eyes, with the forward swept wings, top turret and pointy nose... then again, you could say similar about my suggestions.

Either way the oval wings, rotary engines and tail dragger put the plane in WWII era, a lot of planes were designed during that era that copied from each other, so unless the original designer comes along guesses are all we have

Agreed, Oval-ish wings, Rotary Engines, Mid-wing, tail dragger etc are all pretty common.

What about the Bristol Beaufighter?

That did occur to me, but the Beau is very stub-nosed and has a turret, so doesn't fit too well


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Why is a straight-in approach not allowed to RWY 07 at Cluj (LRCL)?
Here's a VOR approach for runway 07 at Cluj (LRCL).
On the lower left side of the chart you will see the "Straight-in" OCA(H) as N/A and then a "Circling" OCA(H) of 2540 (for category C). There is only a 6 degree offset between the final approach course and the runway, and as far as I can tell below 30 degrees (or maybe 15?) the approach is considered straight-in.
So why are straight-in approaches not allowed but circling ones are?

I know under TERPS you can be within the straight-in cone but not able to get straight-in minima due to excessive descent rates on final (see the VOR/DME into KASE for a classic example of this)

Are you asking this question specifically about the VOR rwy 7 approach that you linked to? Or are you asking this question more generally regarding any straight in approach to runway 7 at LRCL?

@JonathanWalters just for this particular approach, why is there no straight in minima? what regulation/reason prevents a straight in

What is shown on that approach isn't that a straight-in approach to the runway isn't allowed, but rather that there are no straight-in minimums published for that approach.
Why they didn't publish straight-in minima is beyond what I can tell from the chart, although the combination of obstacles near the final and reduced navaid accuracy so close to the VOR seems plausible. ("Didn't feel like it because runway 7 is rarely in use" seems less plausible: once the approach has been constructed, most of the work is done, so adding straight-in mins would be negligible extra work at that point, if the criteria were to allow for them.)
The obvious difference between the VOR approach and the GNSS approach is the allowed altitude you can descend to inside of 3 miles to the runway; a precise final approach course definition (i.e. using GNSS) would allow a narrower obstacle clearance corridor - so when you're on course there are no obstacles reaching to within XX feet vertically for YY feet laterally. The less precise guidance from the VOR probably requires a wider corridor, so if some obstacle to the side of the final intrudes on the wide corridor, its minimums have to be raised to clear the obstacle vertically. Raise them high enough, and the straight-in becomes impractical, so only circling mins are published.
Note that if you got the runway in sight & could make a safe approach and landing using normal rates of descent, you could land straight-in from a circling approach. The key point would be that below the circling MDA, obstacle clearance is no longer provided by the approach and it is the pilot's responsibility to avoid all obstacles visually.


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Why is dual crossfeeding prohibited on DA42?
Diamond 42 AFM says:

The fuel crossfeed function can be tested simultaneously with both
  engines. Proper function can be tested by running the engines for
  approx. 30 seconds with CROSSFEED selected. The operation of both
  engines with both FUEL SELECTORS in CROSSFEED position, other than for
  this test, is prohibited.

I looked at the schematics and both fuels crossfeed pipes are completely independent and separate, including the fuel return lines and heat exchangers. Fuel flow looks almost identical (other than longer pipes) when both selectors are in CROSSFEED. So why is the manual prohibiting dual CROSSFEED ?

Maybe it's not clear from the resolution, but in the original you can see on the left image, that the cross fuel flow passes through all the filters, while the cross return line passes through the heat exchanger (the blue box on the lower right side)

Going to do some more research, but my first read through would indicate that the method of fueling the engines uses 1/3 of the fuel delivered. The other 2/3's gets sent back to the tank and is heated considerably. The crossfeed system is supposed to be used to balance the right and left main tanks only, and the cooler system is not designed to be used in a cross-feed situation for more than absolutely necessary. This is only supposition at this point, and I'm attempting to find more documentation to back it up.

As I also could find no satisfactory explanation within the diagrams, AFM, POH or Maintenance Manual, I sent an email to the technicians at Diamond Aircraft.
This was their response, so far:

Hi Chris, I can only assume that if this procedure became too routine, eventually a pilot would select the second electric boost pump on with both systems in cross feed, in flight.  The high pressure created would damage fuel lines and possibly damage the HPPs for both engines.  That would not be a good day.  You can see the step prior to going to cross feed, Fuel Pumps check Off.  I have asked Austria for an explanation but I think I am close to the mark.
Customer Support,
  Technical Services Specialist

Please note, that the customer service technician actually did include his contact information, but that I have removed it for privacy.

Hmm... I think he might be getting somewhere with the pressure and pumps limitation, but this procedure is also disallowed on DA42's without boost pumps, so there needs to be something more

Still no further response from Diamond at this time.

Well, thank you for the bounty. I'm not sure I agree with it, at least not yet. I was really hoping for an answer from Diamond Austria, but nothing yet.

I checked both POH and Maintenance manual: neither explain why.
POH:

CAUTION Do not operate with both fuel selector valves in CROSSFEED position. Do not take-off with a fuel selector valve in CROSSFEED position.

Maintenance Manual:

B. Fuel Transfer
With an engine running and the fuel selector/shut-off valve set to CROSSFEED the fuel supply and return lines to the related fuel tank are shut-off. The engine fuel pump takes the fuel from the opposite engine fuel tank through the related fuel cross feed line. The fuel returned from the engine is returned to the fuel tank which supplied the fuel.
The unwanted fuel flows back from the engine fuel system through a check valve and the fuel selector/shut-off valve. From the fuel selector/shut-off valve the fuel flows through the related fuel return line and a flexible hose to the fuel cooler. Fuel from the fuel cooler the fuel flows through a flexible hose back into the fuel tank that supplied the fuel.

Interestingly, though (italics are mine):

C. Emergency Operation
In an emergency (for example, an engine failure) you can set the related engine fuel selector/shutoff valve to SHUT-OFF. The fuel selector/shut-off valve has a safety gate to prevent accidental selection of the SHUT-OFF position. You must turn and hold the gate in the open position to set a fuel selector/shut-off to SHUT-OFF. When the engine fuel selector/shut-off valve is set to SHUTOFF no fuel can flow to the related engine. The engine can not run.
The pilot can set the other engine fuel selector/shut-off valve to CROSSFEED and use the fuel from both main fuel tanks to supply the engine which is operating. This maintain a similar fuel level in each main fuel tank.

This last paragraph, and the sentence I put in italics in particular, could indicate that if both engines are set to crossfeed they feed on both taks simultaneously, possibly (and this is speculation on my side) leading to a conflict between the two engines over the fuel and/or possible over/misuse of the feed pumps.
I would like to highlight, though, that this sentence does not seem to correspond to the schematics reported (or shown in the manuals).

Yeah, I think on that last quote they meant "alternate the crossfeed valve and use the fuel from both tanks". The schematics are pretty clear that the crossfeed setting will isolate the other tank and the engine will not be supplied from both.